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Audio Spectrum Analyzer 

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Audio Spectrum Analyzer 

Written By: Alex Hand 


Wire cutter/stripper (1) 


Stereo Cable (2) 
Stereo jack (2) 
Screw Terminal (2) 
LM386 audio op-amp (2) 
LM324 OPAMP (7) 
2N4401 NPN Transistor (3) 
LED. Red (3) 
LED. Yellow (9) 
• LED. Green (13) 

1/4W 390k ohm resistor (3) 
1/4W 270k ohm resistor (3) 
1/4W 180k ohm resistor (1) 
1/4W 56k ohm resistor (3) 
1/4W 22k ohm resistor (3) 
Resistor. 10kQ. 1/4W (24) 
1/4W 5.6k ohm resistor (1) 
1/4W 4.7k ohm resistor (3) 
1/4W 1.2k ohm resistor (1) 

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Pagel of 6 

Audio Spectrum Analyzer 

1/4W 1k ohm resistor (3) 
1/4W560ohm resistor (1) 
1/4W 470 ohm resistor (24) 
1/4W68ohm resistor (1) 
1/4W 10 ohm resistor (1) 
100uF / 25v capacitor (electrolytic) (2) 
10uF / 25v capacitor (electrolytic) (3) 
1uF/ 50v capacitor (electrolytic) (1) 
.47uF / 50v capacitor (electrolytic) (3) 
.22uF / 50v capacitor (electrolytic) (2) 
■01uF/250VAC (ceramic) (2) 
100pF (ceramic) (3) 


As complicated as I thought this would be, this circuit is using the basic fundamentals of 
electronics and algebra. The only hard part is perfecting it and expanding it. 

Click here to see the finished project. 

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Page 2 of 6 

Audio Spectrum Analyzer 

Step 1 — Create an amplifier 

1 - • 1 




m~m ' *r 

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t ; ;*'v7 /-V^^B! 

• The first step of this circuit is to build a 200dB stereo amplifier. (It does not have to be 
stereo but it is recommended. This does not replace an audio mixer.) This also does not 
need to be soldered. I only did so because I would use this over and over again. So if you 
do plan to use it often, it is a good idea to make it permanent. 

• This step requires: LM386 (2/2), Green LED (1/13), Stereo Jack (2/2), Screw Terminals 
(2/2), .22uF capacitor (2/2), 10uF capacitor (2/3), 100uF capacitor (2/2), 470 ohm resistor 
(3/24), 4.7k ohm resistor (2/3). 

• No schematic for this part of the circuit can be given (sorry!) because my design software 
(LTspice) does not have the LM386 listed. (Will not work without an amplifier.) If you 
cannot find a schematic you will have to rely on my horrible descriptive talents in hopes of 
making one. To make a stereo amplifier, it is required that you make two mono-channel 
amplifiers (makes sense, right?) and here's how: 

• Start by placing a LM386 anywhere on the board that has plenty of space. Connect pin 4 
and pin 2 to V- and pin 6 to V+. Next, place a 10uF electrolytic capacitor between pin 1 and 
pin 8. Make sure that the cathode (- side) of the capacitor goes to pin 8. (This gives us our 
200db gain.) 

• Next place a 470Q resistor between V- and pin 3. Then place a 4.7kQ resistor between pin 
3 and one of the AUDIO terminals. Which one doesn't really matter. Next, place a .22uF 
capacitor between pins 5 and 6. And finally place a >100uF capacitor between pin 5 and 
your output. And that should work as a mono-channel amp. 

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Page 3 of 6 

Audio Spectrum Analyzer 

Step 2 — Construct the filters 

R2 22k R4 390k 

• In this step we have three bandpass filters that take in the audio and we only use portions 
of it to create a signal and send it to the next step. 

• This step requires: LM324 (1/7), 10pF capacitor (3/3), .01 uF capacitor (2/2), 1uF capacitor 
(1/1), 10uF capacitor (1/3), 10Q resistor (1/1), 69Q resistor (1/1), 560Q resistor (1/1), 
1.2kQ resistor (1/1) 4.7kQ resistor (1/3), 5.6kQ resistor (1/1), 22kQ resistor (3/3), 180kQ 
resistor (1/1), 390kQ resistor (3/3). 

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Page 4 of 6 

Audio Spectrum Analyzer 

Step 3 — Create the display 

• NOTE! This part of the circuit CAN be replaced by the LM3916 and the LM3916 ONLY! 
With that in mind, the final step is creating the display. This takes the audio and amplifies 
it a little more and then displays it as a bar-graph. You may need to use a wire 

• This step requires: LM324 (6/7), 2N4401 NPN transistor (3/3), .47uF capacitor (3/3), 
Green LEDs (12/13), Yellow LEDs (9/9), Red LEDs (3/3) 470Q resistor (24/24), 1kQ 
resistor (3/3), 10kQ resistor (24/24), 56kQ resistor (3/3), 270kQ resistor (3/3). 

• The schematic given is for only 1/3 of the display. It will need to be made three times. 
Also, this circuit was designed to run on 5V. Increasing the voltage would require you to 
change things around a bit. Use only 5V to get the same end results I have gotten. 

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Page 5 of 6 

Audio Spectrum Analyzer 

Step 4 — Extend the circuit 

Low-Pass filter D 'J * Formula: 



V(fcxfc) or V r (l/(2nR 1 C 1 )) x (l/(2nR 2 C 2 ) 

• These are formulas to change 
values or extend the circuit. You 
can use these to create more 
bands and choose what values to 
use! Make sure you use a 

Every part on the parts list is a part on my circuit board. If you change something, something 
bad (or good) might happen. Working with op-amp band-pass filters, a slight change in the 
values given will throw off the results. HOWEVER: You may not end up using all the parts (e.g., 
screw terminals) because you may not solder your LM386 set on a board. I only included them 
because I did solder them. And they are used for power and signal out. But other than that, 
nothing should be unused or traded out. You might also not use the second audio jack. It is not 
mandatory, but it would be silly if you can't hear the music/sound with your new spectrum 
analyzer. The second jack allows you to plug in your speakers while they are disconnected by 
the first jack. 

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Page 6 of 6